Torrefaction, or the roasting of a biomass such as wood chips and the like, has been used in the past to produce biocoal, which is packaged as a briquette or pellet that can be utilized to replace fossil fuel. The biocoal is characterized by high energy density, homogeneity, is hydrophobic, exhibits no biologic activity and offers improved grindability, making the torrefied product not only coming from a sustainable source such as renewable wood supplies but also is environmentally sound, oftentimes when directly replacing natural coal eliminating CO2 emissions.
In replacing fossil fuels biocoal may be used as a wood power fuel, in steel production, in centralized heating and as a clean stock for production of transportation fuels such as methanol.
As exemplified by the present plant at Stamproy Green, biocoal is produced utilizing a standard commercial dryer that has been modified into a torrefaction reactor. It is noted that the original Carrier dryer was designed to recirculate air for a number of drying applications in order to remove moisture from product. The modification of the Stamproy Green reactor involves modifying the dryer with a perforated plate onto which biomass in the form of wood chips are deposited at one end, and through which hot synthetic wood gas or syngas is passed to roast or torrefy the wood in a convective heating operation. The reactor, fed by an inlet airlock device such as a screw, is vibrated causing the roasted or torrefied wood chips to move down the perforated plate to the exit port where the torrefied material is collected by an outlet airlock device such as a screw. In this manner the reactor remains air tight and the torrefied biomass while roasting produces syngas which is subsequently recirculated through the dryer.
It is noted that in this prior modification of the Carrier dryer there are no moving parts within the reactor and there were very few changes needed to the dryer itself, other than external air locks, to make it suitable for use as a convective torrefaction reactor. The changes in the commercial Carrier dryer to accommodate torrefaction include modifying the above-mentioned perforated plate and to provide an external burner to combust a small amount of excess evolving syngas and an external heat exchanger to then heat the larger volume of recirculated syngas so it can be injected into the bottom of the reactor. The external piping of hot syngas poses a severe fire hazard, especially outside the reactor. Because the reactor is vibratory and the piping in the facility is fixed, numerous flexible connections are used and each poses a safety concern.
It is noted that syngas is extremely flammable and in the presence of oxygen rapidly combusts or could potentially explode. The hotter the syngas the higher the likelihood of explosion. Because the existing oxidizer burner output is at about 800 degrees C., an extremely hazardous situation exists outside the reactor due to the superheated flue gas from this burner. This superheated fluegas is heat exchanged with recirculated syngas from the reactor to ideally produce injected recirculated syngas at 300-400 degrees C., but as high as 500° C., requiring a special heat exchanger that can cope with high differential temperatures of 800° C. on one side and 300° C. on the other side. The control of the energy balance between the two sources is problematic and non linear in nature. This heat exchanger also poses a safety risk and is located outside of the reactor.
It is noted that torrefaction ideally takes place at between 250° C. and 300° C., and is to take place in an oxygen-free environment. Aside from the situation of potential leaks outside the reactor, if there is oxygen leakage into the reactor, the syngas that normally evolves during the torrefaction process can explode causing rupture of the reactor or at the very least a significant thermal runaway event that would damage the equipment. When operating at 300° C. the evolving syngas creates a massive safety risk, with any leak involving flammable hot gas looking for oxygen. Also leakage can cause the release of deadly carbon monoxide. Add to this the risks associated with external burners and 500° C. heated syngas ported to the bottom of the reactor through ductwork, present torrefaction installations are exceedingly dangerous.
The leakage danger in vibrating torrefaction reactors is due to the inherent vibration conveying design that loosens the couplings or deforms the coupling material itself between the reaction chamber which is vibrating and the conduits or duct work which are fixed to pipe the syngas which is evolved in the torrefaction process. The present modified Carrier dryers have as many as 15 ducting conduits coupled to the sides of the reactor. At each of these large pipes or conduits is a gland which is a flexible coupling to be able to absorb the vibration of the reactor, with the flexible joints between the fixed conduits and the vibrating reactor designed to prevent the leakage of oxygen into the reactor and to prevent outflow of gases developed in the reactor. However it has been found that there are frequent failures of these glands making such torrefaction systems unsafe as air/syngas leaks are not only difficult to prevent, but also difficult to detect in this process environment.
The Stamproy Green modification of the standard Carrier dryer for torrefaction thus results in numerous ports of entry for oxygen due to the failure of glands around the conduits utilized to duct the waste products from the torrefaction process either to outside the plant or to be used in a heat recapture process to recycle heated gas to the bottom of the reaction chamber.
As will be appreciated it is important to provide a reaction chamber in which oxygen cannot inadvertently leak in and cause fires or explosions. It has been the experience of torrefaction plants in the past that fire is the major cause of failure of the plant and there is therefore an urgent requirement that such reactors be constructed in a different manner to minimize the possibility of the influx of oxygen that can cause explosion or the venting of dangerous gas components or the exhaust of dangerous gases at these glands. Also there is a requirement to increase the efficiency of the system to be able to better utilize the energy associated with evolved syngas to provide a source of electricity to run the process and to capture useful waste heat.
In addition to the leakage problem noted above, an even further problem with the Stamproy Green plant is thermal runaway. Thermal runaway occurs when reactor temperatures cannot be controlled. This is the result of the inability to sufficiently remove energy from the evolving process. Currently aside from slowing down the biomass feed stock supply which can take as long as 30 minutes to be effective, to kill thermal runaway the entire plant has to be shut down. In terms of production this is unacceptable. Thus there is an urgent need to be able to control thermal runaway by including a process element to remove energy from the system.
More particularly, and as mentioned above, in the Stamproy Green plant syngas which is the evolved product in torrefaction, is coupled to a burner which heats the 300° C. syngas by the burning some of the syngas to create 800° C. fluegas. Additionally, syngas which is not coupled to the burner is collected by utilizing a blower and it is directed into a heat exchanger in which the 300° C. syngas is heated by the 800° C. fluegas output of the burner to provide as high as a 500° C. syngas that is injected into the bottom of the reactor. Thereafter and through convective heating the hot gas passes through the aforementioned perforated plate and into the overlying biomass in the form of wood chips at which point the wood chips are torrefied or roasted.
As stated above, this process includes a large number of inlet and exhaust conduits, which can number as many as 15, each having its own flexible gland at which air and therefore oxygen can enter into the reactor upon gland rupture.
This process is indeed dangerous due to the many conduits associated with the reactor as well as for instance the highly heated burner output which is ducted to the reactor. It will be appreciated that that which is injected into the reactor is not inert gas but rather a highly volatile heated syngas which if mixed with oxygen can result in explosions, fire or thermal runaway.